The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time—see e.g. T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N Garroway et al, IEEE Trans. on Geoscience and Remote Sensing, 39, pp. 1108-1118 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance. NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies.
One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample, and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage. There is also need for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors. Problems associated with such a detector using conventional systems are the decrease in detectability with distance from the detector coil, and the associated equipment needed to operate the system.
A NQR detection system can have one or more coils that both transmit and receive, or it can have coils that solely transmit or solely receive. The transmit, or transmit and receive, coil of a NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample, and results in their producing their characteristic resonance signals that the receive, or transmit and receive, coil detects. NQR signals have low intensity and short duration.
The transmit, receive, or transmit and receive, coil is preferably tunable and has a high quality factor (O). After the RF signal is transmitted, the transmit, receive, or transmit and receive, coil will typically experience ringing, and it must have a rapid recovery time in order for the receive, or transmit and receive, coil to be able to detect the low intensity NQR signal. One method of accomplishing this is to use a Q-damping circuit that is activated to provide a rapid recovery.
A simple Q-damping circuit is shown in FIG. 1. The transmit, receive or transmit and receive, coil 1 is inductively coupled to single loop or coil 2. The Q-damping circuit is comprised of single loop or coil 2, a diode switch 3, a capacitor 4 and a resistor 5. Various other component arrangements can be used between points 6 and 7, such as those shown in Kim, U.S. Pat. No. 6,291,994. The single loop or coil 2 can be a single loop, a solenoid, or a center-taped single loop or solenoid. The diode switch 3 is open when no damping is needed, and closed so that the resistive load can provide the Q-damping when needed.
The transmit, receive, or transmit and receive, coil has typically been made of copper and therefore has a Q of about 102. It is advantageous to use a transmit, receive, or transmit and receive, coil made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a Q of the order of 103-106. AS the recovery time is proportional to Q, the HTS coil has a considerably longer recovery time than a copper coil. An efficient Q-damping circuit is therefore a necessity.
An object of the present invention is to provide an efficient Q-damping circuit for an HTS coil.